Horford has not looked great this year. I'm not too sure if that's straight up due to age, or if he's just taking it easy trying to stay healthy for the playoffs.
Kornet has really shown he fits very well with this team. As a 7'2" guy he moves his feet surprisingly well. He has strong defensive impact around the basket, is an excellent rebounder, and very efficient offensively around the basket.
One thing that makes this decision hard is that Al has a tendency to make game winning plays come playoff time, and there have been times in the playoffs where Kornet has seemed to really struggle and somewhat disappear. This makes me thing that Kornet is probably more valuable in the regular season, while Horford is probably more valuable in the playoffs - though it's hard to say if that will be true again this season given Horford's age and Kornet's increased playoff experience and confidence in his role.
It's hard to put a price on leadership ability. When the team gets down or starts losing focus Al is the type of guy who can come out and calm guys down, get them in the right mindset. Likewise the team last year was I think extra motivated to win because they wanted to get Al a ring, and even in this coming year maybe trying to get a second ring for Al might help give the team that little bit of extra motivation.
I think the Celtics will try their best to bring Kornet back if they are realistically capable of doing so. He knows the system, he's proven, and he's a good locker room guy who everybody seems to love - and Brad seems high on him.
As for Al - I think he'll likely retire after this season. The man is 38 years old, he's had a fantastic career, and he has achieve that ultimate goal now of winning a championship. It made sense for him to stick around for this final year of his contact and try to get a second ring, but once his contract is up there probably isn't THAT much more out there for him to chase after unless he just wants to play a bit longer for the love of the game. If he does come back I expect he'll want to retire a Celtic and will probably sign a vet min deal to stay for another year or two.